1. Field of the Invention
The present invention relates to an electrophotographic image-forming device.
2. Description of the Related Art
One type of conventional electrophotographic image-forming device, such as that disclosed in Japanese unexamined patent application publication No. 2001-183916, has an intermediate transfer belt interposed between photosensitive drums and primary transfer rollers and between a conveyed sheet of recording paper and a secondary transfer roller. After applying a voltage to the primary transfer rollers disposed opposite the photosensitive drums to transfer developer images from the photosensitive drums onto the intermediate transfer belt, the image-forming device applies a voltage to the secondary transfer roller disposed opposite the recording paper to transfer the developer images carried on the intermediate transfer belt to the recording paper, forming an image on the recording paper.
Since a high voltage (transfer bias) is applied to the primary and secondary transfer rollers during the transfer operation, the belt unit, including the intermediate transfer belt, a drive roller for driving the belt, and the like, has a tendency to retain an electric charge. Therefore, the invention disclosed in Japanese unexamined patent application publication No. 2001-183916 grounds the belt unit through an electric-resistive material, such as varistors or resistors, to neutralize the belt, drive roller, and other members targeted for neutralization.
In recent years, image-forming devices have been provided with an optical sensor near the drive roller, which stabilizes the behavior of the belt. The image-forming device reads the density and the like of a resist pattern formed on the belt or developer deposited on the belt and controls operations based on this data.
However, since the drive roller is affected by the transfer bias and carries a high voltage charge, there is a high possibility that the optical sensor disposed near the drive roller will be greatly affected by the high voltage carried on the drive roller. Since the optical sensor cannot accurately detect data when strongly affected by such a high-voltage charge, the data detected by the optical sensor may adversely affect the control of the image-forming device.
Although the charge retained by the belt unit can be quickly removed by grounding the belt unit directly rather than through an electric-resistive material, it is difficult to transfer developer images with stability when the charge is removed too quickly from the belt, running a high risk of not being able to form images with stability.
An image-forming device having a direct tandem system for transferring developer images formed on photosensitive drums directly to a sheet of recording paper rather than through an intermediate transfer belt also has transfer rollers disposed on the side of the conveying belt opposite the photosensitive drums. Hence, an optical sensor disposed near the drive roller driving the conveying belt in such a direct tandem type image-forming device is also greatly affected by the high-voltage charge in the drive roller, and cannot accurately detect data.